2.8 Why are collections grayed out? How can I open them?
tl;dr: Your collections are set to be excluded from the scene.

This is the Blender 2.80 replacement for enabling/disabling layers. Instead of selecting or deselecting a couple of layers in your viewport, you set your collection to be excluded from the viewport.

Go to the Outliner, left-click on one or Shift + left-click on multiple collections, right-click to open the Menu and go to "View Layer" and click "Clear Exclude" - alternatively hit the Shortcut Alt+E while your mouse pointer is within the Outliner.